US President-elect Donald Trump has once again expressed interest in buying Greenland, and Greenland has again stated it is not for sale. In recent weeks, Trump has threatened to take over the Panama Canal and said Canada should be the USA's 51st state. While his claims have earned flak for displaying “territorial ambitions”, Trump is not the first US president to want Greenland, nor is buying new territories unheard of for the US. Why is the huge Arctic island important for the US? When did the US last buy territories, and what efforts has it made in the past to acquire Greenland? We explain in three points.